The present invention relates to a method of draining a fiber web in a wire section of a paper machine, and to apparatus for draining the web in the wire section. Both the method and the apparatus are directed particularly to controlling the dewatering speed in the wire section. The apparatus is included in and the method is performed in a machine in which a fiber suspension is fed from a headbox onto at least one wire screen, called a wire, where the web is dewatered. The machine has at least one headbox, a forming region and a suction region. Upon formation of a web in a wire section of a paper machine, numerous influential factors must be observed to obtain a transverse profile of pulp consistency which is as uniform as possible, a uniform transverse profile of the transverse orientation of the fiber or transverse profile of formation, and in order to assure uniform quality of the surface of the paper.
In a traditional headbox, the width of the outlet slot or slice through which the jet of pulp emerges is controlled over the width of the web so as to establish as uniform a transverse profile of the consistency of the pulp as possible. However, obtaining a uniform transverse profile of the formation is uncertain.
Recently, there has been a considerable advance in controlling the headbox with regard to the consistency of the pulp by mixing a selected ratio of a second fiber suspension of lower concentration, for instance drained off water or white water, in a fiber suspension of higher concentration, while maintaining the total volume of flow constant, regardless of the volume of the stream being mixed in or of either stream. See U.S. application Ser. No. 08/662,980. This considerable improvement enables the transverse profile of the pulp consistency to be adjusted without detrimentally affecting the transverse profile of the fiber orientation. However, this arrangement still has the disadvantage that, despite the constant volume of flow of pulp over the width of the web, non-uniform web formation is obtained.